The prevalence of dizziness, faintness, numbness, and tingling symptoms within panic attacks are suggestive of possible fluctuations in cerebral blood flow. The basilar artery is of particular interest because disturbances in the blood flow of this artery are associated with symptoms that are similar to those experienced during panic attacks. Transcranial doppler ultrasonography permits the assessment of sensitivity of the cerebral blood flow in the basilar artery to physiological stimuli, such as hypercapnia, that are likely to affect cerebral blood flow dynamics. In a pilot study, we assessed the response of the basilar artery to 30 seconds of brief hyperventilation in 14 patients with panic disorder and 8 normal controls. Patients with panic disorder demonstrated a greater decrease in both peak blood flow and mean blood flow rates after hyperventilation than normal controls. The purpose of the present study is to investigate the blood flow response of the basilar artery to brief voluntary hyperventilation, using TCD methodology and capnography, in 30 patients with panic disorder and 30 normal controls. We will examine the association between basilar arterial flow and changes in respiratory physiology as assessed by end-tidal partial pressure of carbon dioxide (pCO2). An additional goal is to examine the distinctiveness of the basilar response by including the middle cerebral artery as a comparison artery. The hypothesis is that patients with panic disorder will demonstrate a greater reduction in basilar arterial blood flow in response to a hyperventilation as compared to controls. An additional hypothesis is that this sensitivity will be demonstrated to occur beyond the effects of changes in respiratory physiology and will be more specific to the basilar arterial system than to the other cranial arterial systems. The long-term goal of the study, is to elucidate a possible mechanism for the development of several primary symptoms occurring within panic attacks and to increase the understanding of the dynamics of cerebral blood flow during anxiety. Further implications of the basilar artery response are discussed in terms of possible considerations in conceptualizing and treating panic disorder.